External tube artery flexible heat pipe

ABSTRACT

A flexible heat pipe employing external tube arteries in the adiabatic region to transfer the heat pipe working fluid from the wick contained in the condenser portion to the wick contained in the evaporator section.

United States Patent n 1 Franklin et al.

[ Oct. 21, 1975 1 EXTERNAL TUBE ARTERY FLEXIBLE HEAT PIPE [75]Inventors: James L. Franklin, Bellevue; John T. Pogson, Seattle, both ofWash.

[73] Assignee: The Boeing Company, Seattle,

Wash.

[22] Filed: Oct. 1, 1973 [21] Appl. N0.: 402,655

[52] US. Cl 165/105; 122/366 [51] Int. Cl. F28D 15/00 [58} Field ofSearch 165/105; 122/366 [56] References Cited UNITED STATES PATENTS3,554,183 1/1971 Grover et al 165/105 X 3,604,503 9/1971 Feldman, Jr. etal 165/105 X 3,661,202 5/1972 Moore. Jr. 165/105 3,670,495 6/19723,734,173 5/1973 Moritz 165/105 Primary Examiner-Albert W. Davis, Jr.Attorney, Agent, or FirmDonald A. Streck 5 7] ABSTRACT A flexible heatpipe employing external tube arteries in the adiabatic region totransfer the heat pipe working fluid from the wick contained in thecondenser portion to the wick contained in the evaporator sectron.

1 Claim, 8 Drawing Figures US. Patent Oct. 21, 1975 Sheet 1 0133,913,665

U.S. Patent Oct. 21, 1975 Sheet 2 of 3 3,913,665

EXTERNAL TUBE ARTERY FLEXIBLE HEAT PIPE BACKGROUND OF THE INVENTION 1.Field of the Invention The present invention relates to heat conductivedevicesand more particularly to heat pipes wherein a wick is employed totransfer a fluid in an evaporativelcondensation cycle through capillaryaction.

2. Description of the Prior Art A heat pipe is a closed environmentcontaining a fluid which constantly undergoes an evaporative/condensation cycle. A continuous wick transfers the condensed fluid fromthe cold portion or condenserto the hot portion or evaporator where thefluid returns to the vapor state. The vapor then moves through theclosed environment in that portion not occupied by the wick base to thecondenser where it returns to the fluid state. If the heat pipe is toremain operative, the integrity of the cycle must be maintained. Loss offluid continuity in the wick is the critical item in the cycle.Typically the wick is constructed of a material or by a process whichwill yield a porous structure comprising a series of intermeshedcapillaries. As the fluid in the evaporator enters the gaseous state ahigh meniscus is formed in these capillaries. The fluid is drawn towardthe evaporator by the surface tension of the meniscus. If a dry spotshould form across the wick the continuity of this fluid flow may belost and the cycle broken. Likewise if the prime of the wick is lost;the cycle may not begin when heat is applied to the evaporator sectionof a heat pipe in the static condition.

The foregoing is particularly important when it is desired toincorporate a flexible portion within the heat pipe as may be requiredin many applications particularly where vibration or body forces i.e.gravity may be a factor. The environmental enclosure of the heat pipecan be made flexible through the use of common materials such asflexible tubing. Providing a flexible wick with adequate performancecharacteristics which will resist forming discontinuities or changes inperformance characteristics is another matter.

The use of helical capillary passages contained within a bellows hasbeen advocated but is limited by surface tension pumping capabilities.Consequently, the total energy that can be dissipated by the heat pipein a gravity environment is small. Likewise, a wire mesh cut on the biashas been used to bridge the discontinuity in the wick across theflexible portion. In this case the fluid flow capacity of the wire meshis adequate but the screen wick tends to pull away from the tubewallcausing an inefficiency and loss of lifting capacity where the condensoris located above the evaporator.

Another feature which would be desirable in a heat pipe is the abilityto provide a simple on/off or diode capability. An externalarteryconducting the working fluid can provide such control. If the capillaryis heated, causing the liquid to vaporize within the capillary, thecycle will stop. When the capillary is cooled, vaporization within theartery cannot take place and the cycle will continue.

Therefore, it is an object of the present invention to provide a highperformance flexible heat pipe of low flow resistance, high resistivityto loss of prime, and high flow capacity.

It is another object of the present invention to provide a highperformance flexible heat pipe which can be constructed of non-specialmaterials.

It is yet another object of the present invention to provide a highperformance flexible heat pipe which allows for the cooling or heatingof any external arteries contained in the structure.

It is a further object of the present invention to pro-' vide a flexibleheat pipe that is self priming.

It is a final object of the present invention to provide a flexible heatpipe that eliminates the need for continuous wicks and permits the useof composite wick concepts.

Other objects and advantages of the present invention will becomeapparent from the figures and specifications which follow.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a heat pipe employing a flexiblesection as disclosed by the present invention wherein a felt metal wickis employed.

FIG. 2 is a heat pipe employing a flexible section as disclosed by thepresent invention wherein a slab artery with spirally grooved walls isemployed.

FIG. 3 is a cross sectional view of FIG. 1 at 3-3.

FIG. 4 is a heat pipe of a slab configuration employing a flexible jointas disclosed by the present invention between a solid evaporator andsolid condenser. In this example the capillary arteries connecting thewicks are of a tortion bar configuration as where minimal flexure isanticipated.

FIG. 5 is an optional configuration for an interconnecting capillaryartery for use with slab heat pipes as in FIG. 4 allowing greaterflexure with less resistance.

FIG. 6 is a partial cross section through a slab heat pipe as shown inFIG. 4 wherein the two slabs are sealed and the flexure interconnectionis replaced with a hinge means. In this configuration, both the wicksand vapor paths are interconnected with capillary arteries.

FIG. 7 is a partial cross section at 7 -7 in FIG. 6 showing thestaggering of the capillary arteries to interface with (the wick andvapor space.

FIG. 8 is a temperature profile for a methanol flexible heat pipe 0.5inches in diameter and 1.5 feet in length.

Note that in FIGS. 1 through 7 like functioning elements are numericallythe same even though sometimes differently shaped so as to easilyinterrelate the various species of heat pipe employing the presentinvention.

DESCRIPTION AND OPERATION OF THE INVENTION The basic heat-pipe assembly10 comprises an evaporator section 12, an adiabatic region 14, and acondenser section 16 as shown in FIG. 1, FIG. 2, and FIG. 4. A liquid(not shown) circulates throughout the assembly 10 carrying heat from theevaporator section 12 to the condenser section 16 as vapor and returningto the evaporator section 12 as a liquid. While the shape of the heatpipe may vary for differing applications as will be describedhereinafter, the basic configuration and operation of the presentinvention will be described in relation to a cylindrical heat pipe asshown in FIG. 1.

Referring to FIG. 1, the rigid portions of the heat pipe are formed byouter enclosures 18. The outer enclosures 18 are completely closedexcept where they interface with a flexible conduit 20 which intercomnects the outer enclosures 18 to form a closed environment assembly 21.Within the outer enclosures l8 and adjacent to the periphery thereof isa porous wick 22 as more clearly shown in FIG. 3. In the preferredembodiment as tested to date the wick 22 is constructed of metal felt.The wick 22 is contained only in the outer enclosures 18 and terminatesat the interfaces with the flexible conduit 20.. The space remainingwithin the closed environment assembly 22 provides a vapor path 24 forliquid vapor (not shown) to flow from the evaporator section '12 to thecondenser section 16 while the wick 22 provides a path for the return ofthe liquid (not shown) from the condenser section 16 to the evaporatorsection 12.

The present invention provides a means for bridging the discontinuity inthe liquid flow path as hereinbefore described due to the absence of thewick 22 in the flexible conduit 20. Capillary artery tubes 26 areoperably connected through the outer enclosures 18 to provide a path forliquid flow from the wick 22in theevaporator section 12 to the wick 22in the condenser section 16. In the preferred embodiment, as depicted indetail in'FIG. 3, an artery structure 28 is contained between the wick22 and the outer enclosure 18 to provide a path for the dispersal of theliquid and reduce pressure loss. The artery structure 28 extends bothlongitudinally and circumferally for optimal liquid transfer. In acylindrical heat pipe as shown in FIG. 1 the capillary artery tubes 26are positioned helically about the flexible' conduit 20 to provideflexibility with minimal single point flexure in the capillary arterytubes 26. The material of the capillary artery tubes 26 is determined asis the material of the entire outer enclosure 18 by the physicalrequirements of containing the liquid used in the heat pipe. The size isdetermined by the application and is a function of the number ofcapillary artery tubes 26 and the pressure differential across theflexible conduit 20. Various configurations employing the presentinvention are described hereinafter.

Referring to F IG. 2, a heat pipe assembly is shown in a'cylindricalform as that of FIG. 1. FIG. 2 demonstrates the interfacing technique tobe employed where a composite wick structure is used such that there isa poor transfer potential between the wick and the capillaries at theirjuncture. As shown in FIG. 2 a slab artery with 'spirally grooved walls30 replaces the conventional wickrln this case an interfacing wick 32 ofmetal felt is provided to connect the slab artery 30 to the capillaryartery 26. The interfacing wick 32 provides a buffer to contain theliquid and allow transfer between the slab artery 30 and the capillaryartery 26 in the optimal manner for each. The interfacing wick 32incorporates an artery structure 28 as described in conjunction withFIG. 1 herein before.

Referring to FIG. 4, a heat pipe assembly 10 is shown in a slab formwith the flexible conduit in the form of a bellows as in an accordion.In the configuration as shown, the capillary arteries 26 are shaped toprovide a tortion bar effect providing stiffness and limitedflexibility. By incorporating capillary arteries 26 as shown in FIG. 5,the same slab heat pipe would be more flexitwo distinct outer enclosures18. The flexible conduit 1 20 of FIGS. 1, 2, and 4 is replaced withhinge means 34. In such an arrangement, if the capillary arteries 26 areof a stiff tortion configuration as shown in FIG. 4, one

slab can be folded and latched in place for subsequent automaticdeployment when unlatched. Since the continuity of the vapor path 24 islost, vapor arteries 36 would have to be provided to interconnect thevapor paths 24 as shown in FIG. 6 and FIG. 7. To prevent condensationwithin the vapor arteries 36 they would have to be surrounded withinsulation 38 and of sufficient number and size to provide full vaporflow. The. same configuration would, of course, work if the hinge means34 were removed and the two slabs were physically separated.

What is claimed is:

1. A heat pipe containing a quantity of fluid and being an airtightenclosure composite structure com-. prising in combination:

a. first enclosure means defining a first space and a second space, saidfirst space containing first internal conductor means for containing andtransporting the fluid therethrough as a liquid, said second space beinga passageway for containing and trans porting the fluid therethrough asa vapor;

b. second enclosure means operatively connected to i said firstenclosure means and having a flexible passageway disposed adjacent tosaid second space of said first enclosure means so as to allow the fluidto move from said second space of said first enclosure means into saidsecond enclosure means as a vapor;

c. third enclosure means comprising a third space and a fourth spaceoperatively connected to said second enclosure means, said third spacecontaining second internal conductor means for containing andtransporting the fluid therethrough as a liquid, said fourth space beinga passageway disposed adjacent to said second enclosure means so as toallow the fluid to move from said second enclosure adjacent to saidfirst internal conductor means and i said second internal conductormeans so as to allow the fluid to move from said second internalconductor means to said first internal conductor means through saidflexible external capillary conduit means, a portion of said flexibleexternal capillary conduit means being in the shape of a helix,

said flexible passageway of said second enclosure means being disposedwithin the coils of said helix.

1. A heat pipe containing a quantity of fluid and being an airtightenclosure composite structure comprising in combination: a. firstenclosure means defining a first space and a second space, said firstspace containing first internal conductor means for containing andtransporting the fluid therethrough as a liquid, said second space beinga passageway for containing and transporting the fluid therethrough as avapor; b. second enclosure means operatively connected to said firstenclosure means and having a flexible passageway disposed adjacent tosaid second space of said first enclosure means so as to allow the fluidto move from said second space of said first enclosure means into saidsecond enclosure means as a vapor; c. third enclosure means comprising athird space and a fourth space operatively connected to said secondenclosure means, said third space containing second internal conductormeans for containing and transporting the fluid therethrough as aliquid, said fourth space being a passageway disposed adjacent to saidsecond enclosure means so as to allow the fluid to move from said secondenclosure means into said fourth space of said third enclosure means asa vapor; and, d. flexible external capillary conduit means operativelyconnected to said first enclosure means and to said third enclosuremeans, said flexible external capillary conduit means having its endsdisposed adjacent to said first internal conductor means and said secondinternal conductor means so as to allow the fluid to move from saidsecond internal conductor means to said first internal conductor meansthrough said flexible external capillary conduit means, a portion ofsaid flexible external capillary conduit means being in the shape of ahelix, said flexible passageway of said second enclosure means beingdisposed within the coils of said helix.